Hearing instruments or hearing aids typically comprises a microphone amplification assembly which includes one or several microphones for receipt of incoming sound such as speech and music. The incoming sound is converted to an electric microphone signal or signals that are amplified and processed in a control and processing circuit of the hearing instrument in accordance with one or more preset listening program(s). These listening programs have typically been computed from a user's specific hearing deficit or loss for example expressed in an audiogram. An output amplifier of the hearing instrument delivers the processed microphone signal to the user's ear canal via a miniature speaker or receiver that may be housed in a casing of the hearing instrument together with the microphone or separately in an ear plug.
The noise-level of an input stage of a microphone amplification circuit is generally critical to an overall noise floor in the hearing aid for example expressed as equivalent input noise level in dB SPL at the microphone. The input stage may comprise a single MOSFET or bipolar transistor with a suitable bias current source that determines a bias current through the MOSFET or bipolar transistor. Since the noise level of the input stage is strongly dependent on the bias current level with increasing noise level at decreasing bias current level, this limits how small the bias current can be without the noise level becomes unacceptably high. At the same time, the input stage must also be able to handle the maximum audio signal level the microphone can output without noticeable distortion which requires the input stage is supplied with a relatively high power supply voltage to accommodate the ac signal swing at the maximum audio signal level. Where the microphone comprises an internal preamplifier, i.e. mounted inside the microphone housing, powered by a certain DC supply voltage delivered by the microphone amplification assembly, it has become normal practice to use a higher DC supply voltage for the input stage of the microphone amplification circuit. This higher DC supply voltage may be about 2 times higher than the supply voltage of the build-in microphone preamplifier. At the same time a relatively large bias current is still required in the input stage of the microphone amplification circuit for the reasons discussed above. This means that the input stage may consume as much as 25% of a total current consumption of the microphone amplification circuit even including analog-to-digital conversion of the amplified microphone signal.
In view of the limited amount of energy stored in typical hearing instrument battery cells, it may be desirable to reduce the power consumption of hearing instrument circuitry and components where and whenever possible. Hence, reducing the power consumption of the input stage of the microphone amplification circuit without compromising noise performance and the ability to handle the maximum audio signal level may be desirable and advantageous.